Updates a contact property's fallback value, the only mutable field. The key and type cannot be changed after creation; create a new property instead. ID of the contact property to update (`prp_`-prefixed). Default used when a contact has no value for this property and the template does not supply an inline fallback. A string, number, or boolean matching the declared type (strings up to 500 characters); a value of another type returns a validation error. Set to null to remove the fallback. ID of the property (`prp_`-prefixed), accepted by every operation that takes a `property_id`. The property key, used as the key in contact data and as the template variable name in broadcasts. Lowercase letters, digits, and underscores, starting with a letter. Cannot be changed after creation. The value type every contact must use for this property. Cannot be changed after creation. Possible values: `string`, `number`, `boolean` Default used when a contact has no value for this property and the template does not supply an inline fallback. A string, number, or boolean matching the declared type (strings up to 500 characters), or null when no fallback is set. Whether the property is archived. An archived property is rejected in new contact writes and stops rendering in templates, but every value already stored on contacts is preserved. Reactivate it with unarchive.